Plural motor control circuit

ABSTRACT

Disclosed is a motor control circuit for simultaneously controlling a plurality of motors, said motors in turn controlling the motion of a translatable point along a desired straight or curved path. A first motor is adapted to move said point along a first axis at a first speed, while a second motor is adapted to move said point along a second axis at a second speed. The motor control circuit controls the relative speed of the first and second motors, thereby determining the velocity and path of the translatable point. As a preferred use of the control circuit, the translatable point is a magnet movable under a nonmagnetic game board. On the top surface of the game board, a toy vehicle or the like is moved by the magnet.

Vinner Aug. 27, 1974 PLURAL MOTOR CONTROL CIRCUIT [75] lnventor: Tihamer S. Vinner, Somerset, NJ.

[73] Assignee: SLM Plastics, Inc., Somerset,

a part interest [22] Filed: Sept. 27, 1972 [21] Appl. No.: 292,617

[52] US. Cl. 318/576, 318/67 [51] Int. Cl. G051) 19/36 [58] Field of Search 318/2, 5, 8, 14, 575, 576, 318/580, 587, 625, 663, 665, 67, 68, 80, 82,

9/1958 Limberger 318/576 X 2/1967 Bonanno 318/295 X Primary Examiner-T. E. Lynch Attorney, Agent, or FirmTheodore E. Galanthay [57] ABSTRACT Disclosed'is a motor control circuit for simultaneously controlling a plurality of motors, said motors in turn controlling the motion of a translatable point along a desired straight or curved path. A first motor is adapted to move said point along a first axis'at a first speed, while a second motor is adapted to move said point'along a second axis at a second speed. The motor control circuit controls the'relative speed of the first and second motors, thereby determining the velocity and path of the translatable point. As a preferred use of the control circuit, the translatable point is a magnet movable under a non-magnetic game board. On the top surface of the game board, a toy vehicle or the like is moved by the magnet.

PLURAL MOTOR CONTROL CIRCUIT BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a motor control circuit and more specifically to a control circuit for simultaneously controlling a plurality of motors, said motors in turn controlling the motion of a point along a desired path.

2. Description of the Prior Art In the prior art, it is generally known to control the motion of a movable point along a desired path. It is also known to utilize two motors such that each of the motors moves the point along one of two perpendicular axes. However, the prior art control circuits generally relate to controlling relatively expensive motors having a high internal resistance. This, in turn, requires high potential sources resulting in relatively high heat generation. A further failing of prior art motor control circuit motor combination has been the inability to move a translatable point in any direction alony any desired path. Consider, for purposes of example, that the translatable point is a magnet movable under a nonmagnetic game board and that a toy vehicle is moved on the top surface of the game board by the magnet. Although such a scheme was previously attempted, the prior art control circuits do not permit complete freedom of motion along any desired curved or straight path. One type of prior art circuit utilizes rotary switches for controlling the relative potential to two motors. This technique permits translation in only the directions specified by the switch positions. Another technique utilizes a circular rheostat contacted by movable wiper contacts to apportion the relative potential to two motors. This latter technique also fails to provide complete freedom of motion because it lacks a provision for the threshold starting current of a motor. This last mentioned technique has the further disadvantage of using excessive power since, current always passes through the entire circular rheostat, in addition to the current required by the motors, again resulting in excessive power consumption and heat generation.

SUMMARY OF THE INVENTION It is accordingly an object of this invention to continuously control the motion of a translatable point with complete freedom of motion along any straight or curved path.

It is another object of this invention to simultaneously control the rotational speed of a plurality of motors, with minimum power consumption.

It is still another object of this invention to remotely control the operation of a movable toy vehicle, or the like.

It is a still further object of this invention to simulate the operation of one or more automobiles.

In accordance with a preferred embodiment of the present invention, the relative speed of rotation of two DC. motors is controlled by a control circuit. The control circuit includes a split-ring rheostat with four movable wiper contacts, each wiper contact being electrically connectable to one of the two inputs of each of the two DC motors. Each of the two halves of the split-ring rheostat is connected to a source of. DC. power by means of a center tap. A first one of the two halves is connected to a positive source of potential while the second half is connected to a negative source of potential. The wiper contacts are spaced from each other by means of wiper arms, which in turn are mechanically attached to a rotatable center shaft. In this manner, the maximum potential is supplied to the center of each rheostat, the potential tapering off towards the gap to meet the motor load and design chracteristics. As the rotatable shaft is rotated, the potential to one motor is gradually increased as the potential to the other motor is decreased. Those skilled in the art will recognize that configurations utilizing more than two sections, or only one section of variable resistance could achieve the desired result by following the herein teachings.

In accordance with other aspects of this invention, it has been found that A.C. motors are also controllable by the principles of the present invention. It has been found that structure other than the split-ring rheostat of the preferred embodiment, may be utilized to control A.C. motors. In addition to toy vehicles, and the like, uses of such a motor control circuit motor combination include remote controlled lawn mowers, driver training devices, etc.

The above mentioned objects, features and advantages of the invention, together with others inherent in the same, are attained by the apparatus illustrated in the drawings, the same being merely preferred exemplary forms, and are described more particularly as follows:

IN THE DRAWINGS FIG. 1 is a schematic representation of apreferred embodiment of the control circuit of this invention, operatively connected.

FIG. 2 is an alternate embodiment.

FIG. 3 is a schematic representation of the control circuit used in the environment of a toy vehicle game.

DESCRIPTION Refer now to FIG. 1 for a description of the preferred embodiment of the control circuit of this invention, operatively connected to move a translatable point. DC power supply 10 is electrically connected to the variable resistance controller as shown. The positive output of power supply 10 is connected to a center tap of the left half of split-ring 12 while the negative output is connected to the center tap of the right half of the splitring resistance 12. At this point, current is prevented from flowing by gaps l4 and 16. The split-ring resistance is contacted by contacts 18, 20, 22 and 24. Each of these wiper contacts is supported at the end of a wiper arm 26, 28, 30 and 32, respectively. The wiper arms are disposed at 90 intervals from each other and are mechanically connected to a rotatable means 34. The wiper arms may be made from a conductive metal but are electrically insulated from each other. As will be more apparent later herein, the rotatable means 34 may be adapted to receive a steering wheel if it is desired to use the control circuit for simulating the operation of a vehicle.

An electrical path is established from the split-ring resistance to each of the two motors to be controlled through conductors 36, 38, 40 and 42 which are connected to wiper arms 26, 28, 30 and 32, respectively. Variable resistance 44 is inserted in the conductive line 36 while variable resistance 46 is inserted in conductive line 38. These resistances 44 and 46 are adjustable simultaneously to retain the same value thereby adjusting a predetermined, desired speed into each of motors 48 and 50. The adjustability of resistance values of variable resistance 44 and 46 is also useful for matching the design characteristics of each of the motors with the resistance 12 and the magnitude of potential provided by power supply 10. It should at this point be recognized that power supply can be a battery as well as the DC output of a power supply pluggable into an AC source. The relative speed of rotation of motors 48 and 50 will move translatable point 52 in any desired curved or straight path as will be described in greater detail.

Refer now to FIG. 2 for an alternate embodiment of the control circuit of this invention. Corresponding reference numerals are utilized for corresponding portions, where practical. This embodiment is more particularly useful if each of the motors are mounted on a vehicle each driving one of two (either left or right) driving wheels. Power supply 10' may be either AC or DC, in this case, and is connected by conductor 11 to the center tap of resistance 12 while conductor 13 connects the other terminal of the power supply to both of motors 48' and 50. Resistance 12' has a single gap 14 therein and a conductive section 15. The conductive section extends for the same angle as the angle-of'separation between wiper arms 26' and 28. Also, each edge of the conductive section is an equal distance from gap 14. The conductive line 11' may therefore contact section 15 anywhere and still be effectively connected at the center tap. Contacts 18' and 20' are adapted to contact this single gapped ring configuration as in the previously described embodiment. The wiper arms 26 and 28 are connected to rotatable means 34' which is adapted to move the contacts and maintain the constant angle between them. Variable resistance 44 is connected in the conductive path 11 for regulating the overall speed of motors 48' and 50. The same function could, of course, be performed by a variable power I supply 10'. An electrical path is established from resistance 12' through contacts 18' and 20', wiper arms 26' and 28, through conductors and 42' to motors 50 and 48', respectively.

Refer now to FIG. 3 which is a schematic representation of the control circuit of the present invention used in the embodiment of a toy vehicle game. The game is played on the top surface of a rigid game board 60 which is supported by enclosing sides 62. The height of sides 62 may be any convenient height for enclosing the control circuit, motors, and translating devices. Typically, a height of six inches will provide the needed package volume and maintain the game conveniently playable on a table top. The control circuit (as shown in FIG. 1, for example) is mounted under the game board and has been generally identified by reference numeral 64. Rotatable means 34" (consistent with the reference numerals 34 and 34 in the previous figures) I is shown separately as a rigid shaft. Attached to this shaft is a steering wheel 35. Motor 48" is also mounted under game board 60 and is attached to move transverse bar 66 in translation in the directions indicated by the arrows. Transverse bar 66 is slidably supported on fixed bar 68 and moved by strings, for example, which in turn are controlled by the rotation of motor 48". The details of specific mechanical interconnections are not described in greater detail as they are known to those skilled in the art. The second motor 50 is mounted on transverse bar 66, and thereby moves with bar 66. Motor 50" controls the motion of translatable point 52' in the direction indicated by the arrows. If the direction of motion provided by motor 50 along transverse bar 66 is designated as an X axis, and the direction of motion indicated for transverse bar 66 in response to motor 48 is designated as a Y axis, then it is apparent that translatable point 52 is movable in any direction as a resultant component of the X and I velocities.

In a preferred form, translatable point 52 is a permanent magnet having north and south poles and movable underneath game board 60. A toy vehicle, or the like, placed on the top surface of game board and having a permanent magnet mounted therein, with dissimilar polarities of the two magnets being in closest proximity to each other, will then follow the motion of the magnet underneath game board 60. For purposes of illustration, game board 60 has been shown transparent in order to permit the schematic representation. It is, however, preferred that game board 60 as well as the sides of the game be opaque. This permits indicia such as roads, highway signs, construction sites, etc. to be painted thereon as well as structures such as houses or the like to be mounted thereon. It is further possible to provide the slidable connection between bars 66 and 68 with a certain amount of vertical freedom of movement providing three dimensional operation. Preferably, the slidable connection having vertical freedom of movement is provided in the mounting of the magnet represented by translatable point 52 thereby permitting the simulation of hills, passing over of bridges and the uneven terrain encountered at construction sites. This will also depend on the type of vehicle it is desired to simulate. Three dimensional operation is also simulated by providing magnets sufficiently powerful to retain magnetic field contact even when the distance between them is increased, such as by a vehicle passing over a hill. Those skilled in the art will also recognize that two completely independent translation systems as illustrated in FIG. 3 may be placed under a single game board 60. As for example, if the second translation system utilizes a significantly stronger magnet than the first system, then it can operate at a lower plane and still control its associated vehicle. It is also preferred to have the magnets of both systems oriented in the same polarity so that the vehicles on the top surface of game board 60 are interchangeable. The magnet in the second system may also have vertical freedom of motion and be held fairly close to the bottom surface of game board 60, as by the mutual attraction of the magnets. The top surface of the magnet structure is convex and polished smooth permitting it to slide under the first translation system with minimum friction. For proper operation of this technique, it is desired that bars 66 and 68 and those bars corresponding to the second translation system be constructed of a non-magnetic material.

OPERATION OF THE INVENTION In operation, the control circuit of the present invention supplies a gradually increasing potential to one of two variable speed motors while supplying a correspondingly gradual decreasing potential to the second one of the two variable speed motors. In this manner, the velocity (speed and direction) of a translatable point is controlled along any desired straight or curved path. As previously mentioned, such an arrangement has application in various fields such as toys, driver training devices, power steering and even electrical cars.

The speed of the two motors 48 and 50 (with reference to FIG. 1) is determined by the rotational position of rotatable means 34. Rotatable means 34 may be actuated manually, as by a steering wheel directly at tached thereto, and also by remote control either electrically, mechanically, or electronically. In the event that rheostat 12 is a split-ring device having two portions as shown in FIG. 1, a complete 360 rotation of rotatable means 34 will cause the vehicle to move in every direction within its freedom of motion. A constant rotation of motor 34 will cause the translatable point to move in a circle. If rheostat 12 were to consist of more segments, then the translatable point would react more quickly. For example, if four segments were used, each alternately supplied with positive and negative polarity potentials, then an 180 rotation of means 34 would move the translatable point in every direction within its freedom of motion. In any event, if means 34 is not rotated at a constant rate, then the translatable point will not move along a constant radius curve. The greater the rotational speed of means 34, the shorter the radius of curvature of the path. In the extreme, when rotatable means 34 is held'sta'tionary, the translatable point moves along a straight path. The absolute velocity of the translatable point is determined by the size of power supply 10, the resistance of rheostat 12, the size of resistance 44 and 46, as well as the design characteristics of motors 48 and 50 and the mechanical ratio of the linkage between the motorsand translatable point 52.

In one embodiment, motors 48 and 50 operate at a variable speed in the potential range between one-half and 3.5 volts. Each half of rheostat 12 has a maximum resistance of 17 ohms from gap 14 to gap 16. The potential supply and resistance 44 and 46 are then adjusted to provide a maximum full load RPM of 3600 for either motor. Each of contacts 18, 20, 22 and 24 has a width slightly less than that of either gap 14 or 16. Thus, when one set of contacts is precisely positioned within the gaps, one of the motors receives no current while the other motor has the maximum potential developed across it, the positions of rheostat l2 immediately adjacent each of the gaps l4 and 16 has a potential equal to the minimum starting potential required to cause rotation of either motor. By this arrangement, it is possible to precisely control the motion of point 52', even in a perfect circle.

By way of specific embodiment, refer to-both FIGS. 1 and 3. Continue with the previous assumption that motor 48 (corresponding to 48" in FIG. 3) moves the translatable point along the Y axis while motor 50 (50" in FIG. 3) moves the point along the X axis. Then, if contact 22 is in gap 14 and contact is in gap 16 no potential is supplied to motor 50. At the same time, contacts 18 and 24 are at the center points (points of maximum potential) causing motor 48 to move at its maximum speed. This will cause the translatable point to move at maximum velocity along the Y axis. Rotating means 34 by an angle of 90 and holding it stationary there will cause the translatable point to move along the X axis at its maximum velocity. Rotating means 34 an additional 90 and holding it stationary there causes translatable point 52 to move along the Y' axis at a maximum velocity in a direction opposite from its previous direction of travel in the Y axis. If means 34 is held stationary in a position such that four contacts are in contact with rheostat 12, then the translatable point moves at a slope determined by the angular position of means 34. If means 34 is rotated at a constant angular velocity, the rotatable point moves in a circle the radius of which is determined by the angular velocity of means 34. The absolute velocity of translatable point 52 may be controlled by variable resistors 44 and 46. These variable resistors are controlled together to simulate the effects of a gas pedal in a toy vehicle game. It is apparent from the foregoing that the resistance characteristics of each section of rheostat 12 in FIG. 1 are linear. It is also apparent that the source of potential supplied at the center of each section must be high enough to meet the minimum starting potential requirements of each motor.

Refer now to FIG. 2 for an alternate embodiment. Assume that motors 48 and are mounted on a vehicle and control the speed of operation of each of two driving wheels. Note that in this embodiment, either AC or DC motors and either an AC or DC power supply may be utilized. Note also that therheostat has only a single gap 14 and includes a conductive section 15. The specific purpose of conductive section 15, as shown is to assure that the speed of only one of themotors 48 or 50 is changed at any one time. This design is preferred because it has been found that controlling the speed of both motors result in undesirably rapid rates of change in the rotation of the two driving wheels. By this embodiment, each of motors 48' and 50' can be used to turn the wheels of a remote controlled electric lawn mower; the direction of rotation of the lawn mower being more easily controlled with this arrangement. Similarly, if motors 48 and 50' were used to control the velocity of the front wheels of a four wheeled vehicle, the FIG. 2 embodiment is preferred. Thus, whenever contact 18 makes contact with conductive portion 15, the speed of motor 50' does not vary. Similarly, when contact 20' is in contact with the conductive section 15, the speed of motor 48' is not varied. The effects of rotating rotatable means 34 on the direction of motion of the vehicle controlled by motors 48 and 50 is thus desirably lessened. Resistance 44 controls the overall speed of the vehicle. This arrangement has been found to be particularly useful in a remote controlled lawn mower in which the angular position of rotatable means 34' is remotely controlled. Those skilled in the art will recognize that in the case of AC power, diode clipping circuits such as those used to vary the brightness of incandesent lights might also be utilized. One such diode clipping circuit would be connected to each of motors 48 and 50' in order to vary the speed of only one motor at any one time; mechanical play could be inserted into the operation of each of the clipping diode circuits.

In conclusion, what has been described is a motor control circuit for controlling the relative speed of rotation of several motors thereby moving a translatable point in any desired curved or straight path. Also, in the event it is desired to rotate the control means many times in the same direction, a commutator or the like is inserted in the circuit path adjacent to the wiper arms to prevent undue twisting of the conductors leading to the motors. Based on the present teachings, numerous modifications, including the rotation of rotatable means 34 by a remote controlled variable speed motor for even more realistic and versatile operation are suggested. The control circuit has been described operating in the specific environment of a toy vehicle game as well as various alternate embodiments. Those skilled in the art will recognize that these as well as various other changes in structure and mode of operation may be made without departing from the spirit and scope of the invention.

What is claimed is:

l. A motor control circuit for controlling a plurality of electric motors, said control circuit comprising:

a split-ring rheostat consisting of two semi-circular resistances adapted to receive a positive potential at the center tap of the first one of said semicircular resistances and a negative potential at the center tap of the second one of said semi-circular resistances;

a first pair of contacts mounted 180 from each other in slidable contact with said rheostat and adapted for electrical connection to the input circuit of a first motor;

a second pair of contacts mounted 180 from each other and perpendicularly to said first pair of contacts, also in slidable contact with said rheostat and adapted for electrical connection to the input circuit of a second motor;

a gap in said split-ring rheostat being substantially the same size as the width of one of said contacts such that no contact being simultaneously in contact with both halves of said rheostat;

mechanical means connected to each said two motors for moving a first magnet movable under a non-magnetic game board; and

means including a second magnet on the top surface of said game board, said second magnet moving in response to said first magnet.

2. A motor control circuit as in claim 1 further comprising:

variable impedance means connected in electrical series with each said first and second motors.

3. .A motor control circuit for controlling a plurality of electric motors, said controlcircuit comprising:

a split-ring rheostat consisting of two semi-circular resistances adapted to receive a positive potential at the center tap of the first one of said semicircular resistances and a negative potential at the center tap of the second one of said semi-circular resistances;

a first pair of contacts mounted 180 from each other in slidable contact with said rheostat and adapted for electrical connection to the input circuit of a first motor;

a second pair of contacts mounted 180 from each other and perpendicularly to said first pair of contacts, also in slidable contact with said rheostat and adapted for electrical connection to the input circuit of a second motor;

a gap in said split-ring rheostatbeing substantially the same size as the width of one of said contacts such that no contact being simultaneously in contact with both halves of said rheostat;

mechanical means connected to each said two motors for moving a translatable point;

a first member fixedly supporting said first motor;

and

a second member fixedly supporting said second motor, said second member being slidably mounted on said first member and movable in a direction parallel to said first member in response to said first motor;

said translatable point being slidably mounted on said second member and movable in a direction perpendicular to said first member in response to said second motor.

4. Apparatus as in claim 3 wherein said translatable point is a first magnet movable under a non-magnetic game board further comprising:

means including a second magnet on the top surface of said game board, said second magnet moving in response to said first magnet.

5. Apparatus as in claim 3 further comprising:

variable impedance means connected in electrical series with each said first and second motors.

6. A motor control circuit for controlling a pair of electric motors, said circuit comprising:

a single circular rheostat with a single insulative gap therein;

a pair of contacts mounted in a fixed relationship to each other and in slidable contact with said single circular rheostat; Y

said single circular rheostat including a conductive portion and being adapted to receive a source of potential at its conductive portion such that when one of said pair of contacts is moved on said conductive portion, a constant potential is tapped, another of said pair of contacts being simultaneously moved on a resistive portion of said rheostat tapping a variable potential;

a pair of electric motors, one each electrically connected to one of said pair of contacts, thereby altering'the speed of only the one of said pair of motors electrically connected to the one of said pair of contacts moved on the resistive portion of said rheostat.

7. Apparatus as in claim 6, further comprising:

said pair of electric motors each adapted to drive one of two drive wheels of a vehicle.

8. A motor control circuit for controlling a plurality of electric motors, said control circuit comprising:

a single split-ring rheostat consisting of two semicircular resistances adapted to receive a positive potential at the center tap of the first one of said semicircular resistances and a negative potential at the center top of the second one of said semi-circular resistances;

a first pair of contacts mounted from each other in slidable contact with said rheostat and adapted for electrical connection to the input circuit of a first motor;

a second pair of contacts mounted 180 from each other and perpendicularly to said first pair of contacts, also in slidable contact with said rheostat and adapted for electrical connection to the input circuit of a second motor;

a gap in said split-ring rheostat being substantially the same size as the width of one of said'contacts such that no contact may simultaneously be in contact with both halves of said rheostat but minimal motion will bring all said contacts into contact with one of the halves of said split-ring rheostat; and

mechanical means connected to each said two motors for moving a translatable point. 

1. A motor control circuit for controlling a plurality of electric motors, said control circuit comprising: a split-ring rheostat consisting of two semi-circular resistances adapted to receive a positive potential at the center tap of the first one of said semi-circular resistances and a negative potential at the center tap of the second one of said semi-circular resistances; a first pair of contacts mounted 180* from each other in slidable contact with said rheostat and adapted for electrical connection to the input circuit of a first motor; a second pair of contacts mounted 180* from each other and perpendicularly to said first pair of contacts, also in slidable contact with said rheostat and adapted for electrical connection to the input circuit of a second motor; a gap in said split-ring rheostat being substantially the same size as the width of one of said contacts such that no contact being simultaneously in contact with both halves of said Rheostat; mechanical means connected to each said two motors for moving a first magnet movable under a non-magnetic game board; and means including a second magnet on the top surface of said game board, said second magnet moving in response to said first magnet.
 2. A motor control circuit as in claim 1 further comprising: variable impedance means connected in electrical series with each said first and second motors.
 3. A motor control circuit for controlling a plurality of electric motors, said control circuit comprising: a split-ring rheostat consisting of two semi-circular resistances adapted to receive a positive potential at the center tap of the first one of said semi-circular resistances and a negative potential at the center tap of the second one of said semi-circular resistances; a first pair of contacts mounted 180* from each other in slidable contact with said rheostat and adapted for electrical connection to the input circuit of a first motor; a second pair of contacts mounted 180* from each other and perpendicularly to said first pair of contacts, also in slidable contact with said rheostat and adapted for electrical connection to the input circuit of a second motor; a gap in said split-ring rheostat being substantially the same size as the width of one of said contacts such that no contact being simultaneously in contact with both halves of said rheostat; mechanical means connected to each said two motors for moving a translatable point; a first member fixedly supporting said first motor; and a second member fixedly supporting said second motor, said second member being slidably mounted on said first member and movable in a direction parallel to said first member in response to said first motor; said translatable point being slidably mounted on said second member and movable in a direction perpendicular to said first member in response to said second motor.
 4. Apparatus as in claim 3 wherein said translatable point is a first magnet movable under a non-magnetic game board further comprising: means including a second magnet on the top surface of said game board, said second magnet moving in response to said first magnet.
 5. Apparatus as in claim 3 further comprising: variable impedance means connected in electrical series with each said first and second motors.
 6. A motor control circuit for controlling a pair of electric motors, said circuit comprising: a single circular rheostat with a single insulative gap therein; a pair of contacts mounted in a fixed relationship to each other and in slidable contact with said single circular rheostat; said single circular rheostat including a conductive portion and being adapted to receive a source of potential at its conductive portion such that when one of said pair of contacts is moved on said conductive portion, a constant potential is tapped, another of said pair of contacts being simultaneously moved on a resistive portion of said rheostat tapping a variable potential; a pair of electric motors, one each electrically connected to one of said pair of contacts, thereby altering the speed of only the one of said pair of motors electrically connected to the one of said pair of contacts moved on the resistive portion of said rheostat.
 7. Apparatus as in claim 6, further comprising: said pair of electric motors each adapted to drive one of two drive wheels of a vehicle.
 8. A motor control circuit for controlling a plurality of electric motors, said control circuit comprising: a single split-ring rheostat consisting of two semicircular resistances adapted to receive a positive potential at the center tap of the first one of said semi-circular resistances and a negative potential at the center top of the second one of said semi-circular resistances; a first pair of contacts mounted 180* from each other in slidable contact with said rheostat and adapted for electrical connectIon to the input circuit of a first motor; a second pair of contacts mounted 180* from each other and perpendicularly to said first pair of contacts, also in slidable contact with said rheostat and adapted for electrical connection to the input circuit of a second motor; a gap in said split-ring rheostat being substantially the same size as the width of one of said contacts such that no contact may simultaneously be in contact with both halves of said rheostat but minimal motion will bring all said contacts into contact with one of the halves of said split-ring rheostat; and mechanical means connected to each said two motors for moving a translatable point. 